c# forms outside the bounds of the array

When I want to receive 3 values from the database.

16.5
15.5
15.5

With this code:

public static double calculateTotalPrice(int reserveringId)
{
    double total=0;
    int i = 0;

    SqlCommand comm =  sqlCrud.returnSqlCommand("select g.prijs from gerechten g inner join besteld b on b.gerechtId=g.gerechtId where b.reserveringId='"+reserveringId+"'");

    SqlDataReader dt = comm.ExecuteReader();

    if (dt.HasRows)
    {
        while (dt.Read())
        {
            total += dt.GetDouble(i);
            i++;
        }
    }

    return total;
}

It's telling me:

Additional information: Index was outside the bounds of the array.

What am I doing wrong?

Thanks